Chemicals including fertilizers may be purchased in either solid granular form or bulk liquid form. The bulk liquid form of the substances (liquid in large drums, by way of example) has inherent problems associated with handling and spillage which generally make the substance cumbersome and hazardous with which to work. A safer alternative to liquid substances is the use of particulate solids which can be dissolved in a liquid to form a solution. The use of particulate solids avoids the hazards associated with spillage of liquid when decanting.
When dissolving a particulate solid in a liquid, it is most advantageous to form a solution of homogenous concentration. The homogenous concentration of the solution is important where specific concentration of a liquid solution is required, by way of example, in chemical dosing for pH control in an effluent plant. Furthermore, where solids have coagulated or not otherwise dissolved, blockage may result downstream of an apparatus, for example, in spray nozzles, pumps, filters and chemical applicators such as boom sprays.
Often solids are dissolved in liquids by adding the solid to the liquid and agitating the combined solution. This agitation may be performed by mechanical stirring or bubbling air through the solution. In both instances, it is difficult to ensure that there is a consistent level of agitation throughout the solution. Consequently, the solids may settle at the bottom of a vessel without dissolving and can coagulate to form a solid or sludge which is difficult to dissolve, and/or can block downstream equipment.
A partial solution to this problem has been proposed in U.S. Pat. No. 2,997,373 to Stephens where the use of a dissolving apparatus is disclosed which comprises a vessel with a cylindrical inlet portion and a conical frustum outlet portion. A pump is in fluid communication with the outlet for recirculating liquid through the vessel. A valve is provided to tap off a portion of the recirculated liquid. The recirculated liquid is redirected into the cylindrical portion and directed to flow in a circular path along the longitudinal axis of the vessel. A number of baffle plates is provided within the vessel, for producing some turbulent action to avoid localization of solids within the liquid. Additionally, Stephens '373 discloses the use of a foraminous member between the cylindrical portion and the conical frustum portion to prevent air from being drawn down into the liquid which may interfere with the operation of the apparatus and cause foaming of the liquid. The foraminous member also acts as a sieve to prevent large undissolved particles from entering the pump.
There is a problem with the use of the baffle plates. They can be detrimental to the overall working efficiency of a system for dissolving particulate solids due to a creation of relatively static pockets of liquid behind the plates and adjacent the wall of a cylindrical portion. Any particulate solids in these pockets are unlikely to be dissolved and will fall to the bottom of the vessel. Typically in the art, such as described in the Stephens '373 patent, the particulate solids are allowed to pass through the pump to be ground by a pump impeller. Depending on the nature of the particulate solid, this can be particularly detrimental to the operation of the pump and can cause very rapid and premature wearing of the pump. It is also believed that the inclusion of the foraminous member, such as described in Stephens, can also interfere with the flow of the liquid, slowing down the speed of undissolved particulate solids in a liquid, and thereby causing them to settle to the bottom and pass through the pump undissolved.